Highly dispersible reinforcing polymeric fibers

ABSTRACT

Synthetic polymer reinforcing fibers provide dispersability and strength in matrix materials such as concrete, masonry, shotcrete, and asphalt. The individual fiber bodies, substantially free of stress fractures and substantially non-fibrillatable, have generally quadrilateral cross-sectional profiles along their elongated lengths. Preferred fibers and matrix materials having such fibers demonstrate excellent finishability in addition to dispersion and toughness properties.

This is a Continuation-in-part (CIP) of U.S. Ser. No. 09/843,427 filedApr. 25, 2001, which is now allowed.

FIELD OF THE INVENTION

The invention relates to fibers for reinforcing matrix materials, andmore particularly to a plurality of synthetic polymer fibers havingexcellent dispersibility and reinforcibility properties, and preferablyexcellent finishability, in hydratable cementitious compositions.Individual fiber bodies are elongated and highly bendable, withgenerally quadrilateral cross-sectional profiles, thereby minimizingfiber balling and maximizing fiber bond.

BACKGROUND OF THE INVENTION

Although fibers of the present invention are suitable for reinforcingvarious matrix materials, such as adhesives, asphalts, composites,plastics, rubbers, etc., and structures made from these, the fibers thatwill be described herein are especially suited for reinforcinghydratable cementitious compositions, such as ready-mix concrete,precast concrete, masonry concrete (mortar), shotcrete, bituminousconcrete, gypsum compositions, gypsum- and/or Portland cement-basedfireproofing compositions, and others.

A major purpose of the fibers of the present invention is to reinforceconcrete, e.g., ready-mix, shotcrete, etc., and structures made fromthese. Such matrix materials pose numerous challenges for those whodesign reinforcing fibers.

Concrete is made using a hydratable cement binder, a fine aggregate(e.g., sand), and a coarse aggregate (e.g., small stones, gravel). Amortar is made using cement binder and fine aggregate. Concretes andmortars are hence brittle materials. If a mortar or concrete structureis subjected to stresses that exceed its maximum tensile strength, thencracks can be initiated and propagated therein. The ability of thecementitious structure to resist crack initiation and crack propagationcan be understood with reference to the “strength” and “fracturetoughness” of the material.

“Strength” relates to the ability of a cement or concrete structure toresist crack initiation. In other words, strength is proportional to themaximum load sustainable by the structure without cracking and is ameasure of the minimum load or stress (e.g., the “critical stressintensity factor”) required to initiate cracking in that structure.

On the other hand, “fracture toughness” relates to the specific“fracture energy” of a cement or concrete structure. This concept refersto the ability of the structure to resist propagation —or widening—of anexisting crack in the structure. This toughness property is proportionalto the energy required to propagate or widen the crack (or cracks). Thisproperty can be determined by simultaneously measuring the load requiredto deform or “deflect” a fiber-reinforced concrete (FRC) beam specimenat an opened crack and the amount or extent of deflection. The fracturetoughness is therefore determined by dividing the area under a loaddeflection curve (generated from plotting the load against deflection ofthe FRC specimen) by its cross-sectional area.

In the cement and concrete arts, fibers have been designed to increasethe strength and fracture toughness in reinforcing materials. Numerousfiber materials have been used for these purposes, such as steel,synthetic polymers (e.g., polyolefins), carbon, nylon, aramid, andglass. The use of steel fibers for reinforcing concrete structuresremains popular due to the inherent strength of the metal. However, oneof the concerns in steel fiber product design is to increase fiber “pullout” resistance because this increases the ability of the fiber todefeat crack propagation. In this connection, U.S. Pat. No. 3,953,953 ofMarsden disclosed fibers having “J”-shaped ends for resisting pull-outfrom concrete. However, stiff fibers having physical deformities maycause entanglement problems that render the fibers difficult to handleand to disperse uniformly within a wet concrete mix. More recentdesigns, involving the use of “crimped” or “wave-like” polymer fibers,may have similar complications, depending on the stiffness of the fibermaterial employed.

Polyolefin materials, such as polypropylene and polyethylene, have beenused for reinforcing concrete and offer an economic advantage due torelative lower cost of the material. However, these polyolefinicmaterials, being hydrophobic in nature, resist the aqueous environmentof wet concrete. Moreover, the higher amount of polyolefin fibers neededin concrete to approximate the strength and fracture toughness of steelfiber-reinforced concrete often leads to fiber clumping or “balling” andincreased mixing time at the job site. This tendency to form fiber ballsmeans that the desired fiber dosage is not achieved. The correctconcentration of fibers is often not attained because the fiber ballsare removed (when seen at the concrete surface) by workers intent onachieving a finished concrete surface. It is sometimes the case thatlocations within the cementitious structure are devoid of thereinforcing fibers entirely. The desired homogeneous fiber dispersion,consequently, is not obtained.

Methods for facilitating dispersion of fibers in concrete are known. Forexample, U.S. Pat. No. 4,961,790 of Smith et al. disclosed the use of awater-soluble bag for introducing a large number of fibers into a wetmix. U.S. Pat. No. 5,224,774 of Valle et al. disclosed the use ofnon-water-soluble packaging that mechanically disintegrated upon mixingto avoid clumping and to achieve uniform dispersal of fibers within theconcrete mix.

The dispersal of reinforcing fibers could also be achieved throughpackaging of smaller discrete amounts of fibers. For example, U.S. Pat.No. 5,807,458 of Sanders disclosed fibers that were bundled using acircumferential perimeter wrap. According to this patent, the continuityof the peripheral wrapping could be disrupted by agitation within thewet concrete mix, and the fibers could be released and dispersed in themix.

On the other hand, World Patent Application No. WO 00/49211 of Leon(published Aug. 8, 2000) disclosed fibers “packeted” together butseparable when mixed in concrete. A plurality of fibers wereseparably-bound together, such as by tape adhered to cut ends of thefibers, thereby forming a “packet.” Within a wet cementitious mix, thepackets could be made to break and/or dissolve apart to permitseparation and dispersal of individual fibers within the mix.

The dispersal of reinforcing fibers may also be achieved by alteringfibers during mixing. For example, U.S. Pat. No. 5,993,537 of Trottieret al. disclosed fibers that progressively fibrillated upon agitation ofthe wet concrete mix. The fibers presented a “low initial surface area”to facilitate introducing fibers into the wet mix, and, upon agitationand under the grinding effect of aggregates in the mix, underwent“fibrillation,” which is the separation of the initial low-surface-areafibrous material into smaller, individual fibrils. It was believed thathomogeneous fiber distribution, at higher addition rates, could therebybe attained.

A novel approach was taught in U.S. Pat. No. 6,197,423 of Rieder et al.,which disclosed mechanically-flattened fibers. For improved keyingwithin concrete, fibers were flattened between opposed rollers to attainvariable width and/or thickness dimensions and stress-fracturesperceivable through microscope as discontinuities and irregular andrandom displacements of polymer on the surface of the individual fibers.This microscopic stress fracturing was believed to improve bondingbetween cement and fibers, and, because the stress-fractures werenoncontinuous in nature, the fibers were softened to the point at whichfiber-to-fiber entanglement (and hence fiber balling) was minimized oravoided. The mechanical-flattening method of Rieder et al. was differentfrom the method disclosed in U.S. Pat. No. 5,298,071 of Vondran, whereinfibers were interground with cement clinker and retained cementparticles embedded into the surface.

In this vein, the nature of the fiber surface has also been a frequenttopic of research in fiber dispersion and bonding in concrete. Forexample, U.S. Pat. No. 5,753,368 of Hansen disclosed a list of wettingagents such as emulsifiers, detergents, and surfactants to render fibersurfaces more hydrophilic and thus more susceptible to mixing in wetconcrete. On the other hand, U.S. Pat. No. 5,753,368 of Berke et al.taught that the bonding between concrete and fibers could be enhanced byemploying particular glycol ether coatings instead of conventionalwetting agents that tended to introduce unwanted air at thefiber/concrete interface.

Of course, as mentioned in U.S. Pat. Nos. 5,298,071 and 6,197,423 asdiscussed above, physical deformation of the fiber surface was alsobelieved to improve the fiber-concrete bond. U.S. Pat. No. 4,297,414 ofMatsumoto, as another example, taught the use of protrusions and ridgesto enhance bond strength. Other surface treatments, such as the use ofembossing wheels to impose patterns on the fiber, were also used forimproving fiber-concrete bond. Fiber designers have even bent fibersinto sinusoidal wave shapes to increase the ability of fibers to resistbeing pulled out from concrete. However, the present inventors realizedthat increased structural deformations in the fiber structure mayactually enhance opportunities for unwanted fiber balling to occur.

Against this background, the present inventors see a need for novelpolymeric synthetic reinforcing fibers having ease of dispersibility inconcrete so as to avoid fiber balling and to achieve intended fiberdosage rates, while at the same time to provide strength and fracturetoughness in matrix materials and particularly brittle materials such asconcrete, mortar, shotcrete, gypsum fireproofing, and the like.

SUMMARY OF THE INVENTION

In surmounting the disadvantages of the prior art, the present inventionprovides highly dispersible reinforcing polymer fibers, matrix materialsreinforced by the fibers, and methods for obtaining these. Exemplaryfibers of the invention provide ease of dispersibility into, as well asstrength and fracture toughness when dispersed within, matrix materials,particularly brittle ones such as concrete, mortar, gypsum or Portlandcement-based fireproofing, shotcrete, and the like.

These qualities are achieved by employing a plurality of individualfiber bodies having an elongated length defined between two opposingends, the bodies having a generally quadrilateral cross-sectional shapealong the elongated length of the fiber body. The individual fibersthereby have a width, thickness, and length dimensions wherein averagewidth is 1.0-5.0 mm and more preferably 1.3-2.5 mm, average thickness is0.1-0.3 mm and more preferably 0.15-0.25 mm., and average length is20-100 mm and more preferable 30-60 mm. In preferred embodiments,average fiber width should exceed average fiber thickness by at least 4times (i.e., a ratio of at least 4:1) but preferably average widthshould not exceed average thickness by a factor exceeding 50 times(50:1). More preferably, the width to thickness ratio of the fibers isfrom 5 to 20 (5:1 to 20:1).

While individual fiber bodies of the invention may optionally beintroduced into and dispersed within the matrix material as a pluralityof separate pieces or separable pieces (i.e. fibers in a scored orfibrillatable sheet, or contained within a dissolvable ordisintegratable packaging, wrapping, packeting, or coating) the fiberscan be introduced directly into a hydratable cementitious compositionand mixed with relative ease to achieve a homogeneous dispersal therein.Individual fiber bodies themselves, however, should not be substantiallyfibrillatable (i.e. further reducible into smaller fiber units) afterbeing subjected to mechanical agitation in the matrix composition to theextent necessary to achieve substantially uniform dispersal of thefibers therein.

Exemplary individual fiber bodies of the invention are alsosubstantially free of internal and external stress fractures, such asmight be created by clinker grinding or mechanical flattening. Thegeneral intent of the present inventors is to maintain integrity of theindividual fiber bodies, not only in terms of structural fiberintegrity, but also integrity and uniformity of total surface area andbendability characteristic from one batch to the next.

A generally quadrilateral cross-sectional profile provides a highersurface area to volume ratio (S_(a)/V) compared to round or ovalmonofilaments comprising similar material and having a diameter ofcomparable dimension. The present inventors believe that a quadrilateralcross-sectional shape provides a better flexibility-to-volume ratio incomparison with round or elliptical cross-sectional shapes, and, moresignificantly, this improved flexibility characteristic translates intobetter “bendability” control. The individual fiber bodies of theinvention will tend to bend predominantly in a bow shape withcomparatively less minimal twisting and fiber-to-fiber entanglement,thereby facilitating dispersion. In contrast, for a given materialelastic modulus and cross-sectional area, the prior art fibers havingcircular or elliptical cross section with major axis/minor axis ratiosof less than 3 will have greater resistance to bending, thereby having agreater tendency for fiber balling when compared to fibers of generallyquadrilateral (e.g., rectangular) cross-section.

The present inventors further believe that a generally quadrilateralcross-section will provide excellent fiber surface area and handabilitycharacteristics when compared, for example, to round or ellipticalfibers. In this connection, preferred fibers of the invention have a“bendability” in the range of 20 (very stiff) to 1300 (very bendable)milli Newton⁻¹*meter⁻² (mN⁻¹*m⁻²), and more preferably in the range of25 to 500 milli Newton⁻¹*meter⁻². As used herein, the term “bendability”means and refers to the resistance of an individual fiber body toflexing movement (i.e. to force that is perpendicular to thelongitudinal axis of the fiber) as measured by applying a load to oneend of the fiber and measuring its relative movement with respect to theopposite fiber end that has been secured, such as within a mechanicalclamp or vice, to prevent movement. Thus, a fiber can be called morebendable if it requires less force to bend it to a certain degree. Thebending flexibility of a fiber is a function of its length, shape, thesize of its cross-section, and its modulus of elasticity. Accordingly,the bendability “B” of the fiber is expressed in terms of milliNewton⁻¹*meter⁻² (mN⁻¹*m⁻²) and is calculated using the followingformula $B = \frac{1}{3 \cdot E \cdot I}$

wherein “E” represents the Young's modulus of elasticity (Giga Pascal)of the fiber; and “I” represents the moment of inertia (mm4) of theindividual fiber body. A fiber having a lower bendability “B” will ofcourse be less flexible than a fiber having a higher bendability “B.”The moment of inertia “I” describes the property of matter to resist anychange in movement or rotation. For a cross-sectional profile having agenerally quadrilateral (or approximately rectangular) shape, the momentof inertia can be calculated using the formula

I _(rectangle)={fraction (1/12)}·w·t ³

wherein “w” represents the average width of the rectangle and “t”represents the average thickness of the rectangle.

In further exemplary embodiments, the “bendability” of fibers can befurther improved if the thickness and/or the width of the fibers arevaried along the length of the fibers, for example from 2.5-25 percentmaximum deviation from the average thickness or width value. This smallvariation of the thickness and/or the width of the fiber also improvesthe bond between the reinforcing matrix and the fiber.

The inventors realized, in view of the above equation for “bendability”“B” of fibers having generally quadrilateral cross-sections, that anincrease in the fiber modulus of elasticity “E” will result in acorresponding decrease in bendability and, consequently, make fiberdispersibility more difficult. The inventors then realized that tomaintain the same level of bendability, the moment of inertia “I” mustbe decreased, and this could be achieved, for example, by reducing thethickness of the fibers while maintaining the cross-sectional area ofthe fibers.

In further embodiments of the invention, preferred individual fiberbodies have the following properties when measured in the longitudinaldimension (end to end) along the axis of the fiber body: a Young'smodulus of elasticity of 3-20 Giga Pascals and more preferable 5-15 GigaPascals, a tensile strength of 350-1200 Mega Pascals and more preferable400-900 Mega Pascals, and a minimum load carrying capacity in tensionmode of 40-900 Newtons more preferable 100-300 Newtons.

A particularly preferred method for manufacturing the fibers is tomelt-extrude the polymeric material (e.g., polypropylene as a continuoussheet); to decrease the temperature of this extruded sheet melt belowambient temperature (e.g., below 25° C.); to cut or slit the sheet(after cooling) into separate or separable individual fiber bodieshaving generally quadrilateral cross-sections to stretch the individualfibers by at least a factor of 10-20 and more preferably between 12-16,thereby to achieve an average width of 1.0-5.0 mm and more preferably1.3-2.5 mm and an average thickness of 0.1-0.3 mm and more preferably0.15-0.25 mm; and to cut the fibers to obtain individual fiber bodieshaving an average fiber length of 20-100 mm and more preferably between30-60 mm. Further exemplary processes are described hereinafter.

The present invention is also directed to matrix materials, such asconcrete, mortar, shotcrete, asphalt, and other materials containing theabove-described fibers, as well as to methods for modifying matrixmaterials by incorporating the fibers into the matrix materials.

Still further exemplary fibers and matrix materials (such as concrete)having such fibers embedded therein are especially suited forapplications wherein “finishability” is important (such as flooringapplications). The term “finishability” refers to the ability of thefibers to resist “pop-up” from the concrete after its surface has beensmoothed over (i.e. “finshed”). The inventors discovered thatfinishability, similar to dispersion, is a function of fiberbendability, but in addition finishibality is also a function of fiberlength. Exemplary fibers having “finishability” are substantially freeof stress fractures and substantially non-fibrillatable whenmechanically agitated within the matrix material, and they have anaverage bendability of 100 to 2,500, and, more preferably, 150 to 2,000mN⁻¹*m⁻². Preferred fibers with finishability characteristics preferablyhave a Young's modulus of elasticity in the range of 4-20 GigaPascals, atensile strength of 400-1,600 MegaPascals, average width of 1.0-5.0 mm,average thickness of 0.05-0.2 mm, and average length of 20-75 mm,wherein average width exceeds average thickness by a factor of 5-50 andmore preferably by a factor of 7-40.

Further advantages and features of the invention are further describedin detail hereinafter.

BRIEF DESCRIPTION OF DRAWING

An appreciation of the advantages and benefits of the invention may bemore readily comprehended by considering the following writtendescription of preferred embodiments in conjunction with theaccompanying drawings, wherein

FIGS. 1-3 are microphotographic enlargements of the cross-sections ofPRIOR ART reinforcing fibers;

FIGS. 4 and 5 are microphotographic enlargements of the generallyquadrilateral cross-sectional profile of exemplary fibers of the presentinvention;

FIG. 6 is microphotographic enlargement (at 25× magnification) of thesurface of an exemplary individual fiber body of the present inventionbefore mixing in a concrete mixture (which would contain fine and coarseaggregates), and FIG. 7 shows the fiber after mixing;

FIG. 8 is microphotographic enlargement (at 200× magnification) of thesurface of an exemplary individual fiber body of the present inventionbefore mixing in a concrete mixture (which would contain fine and coarseaggregates), and FIG. 9 shows the fiber after mixing;

FIG. 10 is microphotographic enlargement (at 900× magnification) of thesurface of an exemplary individual fiber body of the present inventionbefore mixing in a concrete mixture (which would contain fine and coarseaggregates), and FIG. 11 shows the fiber after mixing;

FIG. 12 is a microphotographic enlargement (at 900× magnification) of aPRIOR ART fiber mechanically flattened in accordance with U.S. Pat. No.6,197,423;

FIG. 13 is a graphic representation of tensile load versus strainbehavior of different fibers;

FIG. 14 is a graphic representation of tensile stress versus strainbehavior of different fibers;

FIG. 15 is a photographic of a wedge-splitting device for testing loadon cementitious matrix materials containing reinforcing polymer fibers;

FIG. 16 is a graphic representation of stress vs. crack mouth openingdisplacement behavior of different fibers; and

FIG. 17 is a typical stress versus strain curve of a material forpurposes of illustration of principles discussed herein.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present inventors believe that the reinforcing polymer fibers of thepresent invention may be used in a variety of compositions and materialsand structures made from these. The term “matrix materials” therefore isintended to include a broad range of materials that can be reinforced bythe fibers. These include adhesives, asphalt, composite materials (e.g.,resins), plastics, elastomers such as rubber, etc., and structures madetherefrom.

Preferred matrix materials of the invention include hydratablecementitious compositions such as ready-mix concrete, precast concrete,masonry mortar and concrete, shotcrete, bituminous concrete,gypsum-based compositions (such as compositions for wallboard), gypsum-and/or Portland cement-based fireproofing compositions (for boards andspray-application), water-proofing membranes and coatings, and otherhydratable cementitious compositions, whether in dry or wet mix form.

A primary emphasis is placed upon the reinforcement of structuralconcrete (e.g., ready-mix concrete, shotcrete). However, in general,concrete (whether poured, cast, or sprayed) is an extremely brittlematerial that presents challenges in terms of providing reinforcingfibers that (1) can be successfully introduced into and mixed in thismatrix material and (2) can provide crack-bridging bonding strength inthe resultant fiber reinforced concrete structure.

Prior to a detailed discussion of the various aforementioned drawingsand further exemplary embodiments of the invention, a brief discussionof definitions will be helpful to facilitate a deeper understanding ofadvantages and benefits of the invention. As the fibers of the inventionare envisioned for use in the paste portion of a hydratable wet “cement”or “concrete” (terms which may sometimes be used interchangeablyherein), it is helpful to discuss preliminarily the definitions of“cement” and “concrete.”

The terms “paste,” “mortar,” and “concrete” are terms of art: pastes aremixtures composed of a hydratable cementitious binder (usually, but notexclusively, Portland cement, masonry cement, or mortar cement, and mayalso include limestone, hydrated lime, fly ash, blast furnace slag,pozzolans, and silica fume or other materials commonly included in suchcements) and water; mortars are pastes additionally including fineaggregate (e.g., sand); and concretes are mortars additionally includingcoarse aggregate (e.g., gravel, stones). “Cementitious” compositions ofthe invention thus refer and include all of the foregoing. For example,a cementitious composition may be formed by mixing required amounts ofcertain materials, e.g., hydratable cementitious binder, water, and fineand/or coarse aggregate, as may be desired, with fibers as describedherein.

Synthetic polymer fibers of the invention comprise at least one polymerselected from the group consisting of polyethylene (including highdensity polyethylene, low density polyethylene, and ultra high molecularweight polyethylene), polypropylene, polyoxymethylene, poly(vinylidinefluoride), poly(methyl pentene), poly(ethylene-chlorotrifluoroethylene),poly(vinyl fluoride), poly(ethylene oxide), poly(ethyleneterephthalate), poly(butylene terephthalate), polyamide, polybutene, andthermotropic liquid crystal polymers. A preferred synthetic polymer ispolypropylene. Exemplary individual fiber bodies of the invention maycomprise 100% polypropylene, or, as another example, they may comprisepredominantly polypropylene (e.g., at least 70-99%) with the remaindercomprising another polymer (such as high density polyethylene, lowdensity polyethylene) or optional fillers, processing aids, and/orwetting agents, such as are conventionally used in the manufacture ofpolymer fibers.

The molecular weight of the polymer or polymers should be chosen so thatthe polymer is melt processable. For polypropylene and polyethylene, forexample, the average molecular weight can be 5,000 to 499,000 and ismore preferably between 100,000 to 300,000. Different grades ofpolyethylene may be used, including ones containing branches andcomonomers such as butene, hexene, and octene, and further including theso-called “metallocene” polyethylene materials. If polypropylene polymeris used, it is preferred that no more than about 30 weight percentpolymerized comonomer units or blended resins be present in order tomaintain smooth process operation, with up to about 10% being preferred.Propylene homopolymer resins are most preferred, with general-purposeresins in the nominal melt flow range of about 1 to about 40 grams/10minutes (ASTM D2497 1995). Preferred resins also have weight averagemolecular weight to number average molecular ratios of about 2:1 toabout 7:1.

FIG. 1 is a cross-sectional view, originally taken at about 100×magnification, of a PRIOR ART polypropylene fiber with an ellipticalcross-section having dimensions of 0.96 mm in width and 0.63 mm inthickness. The width is close to thickness, and the fiber can twistalmost equally well in all directions about its longitudinal axis.

FIG. 2 is a cross-sectional view, originally taken at about 100×magnification, of a PRIOR ART elliptical (or oval)-shaped fiber madefrom polyvinylacetate having 0.78 mm width and 0.42 mm thickness.

FIG. 3 is a cross-sectional view, originally taken at about 100×magnification, of a PRIOR ART fibrillatable fiber commercially availableunder the tradename GRACE® Structural Fibers. This fiber is designed tofibrillate or break into smaller fibrils when mixed in concrete. Thecross-sectional profile resembles a tri-lobed peanut.

FIG. 4 is a cross-sectional view, originally taken at about 100×magnification, of an exemplary individual fiber body of the presentinvention. The generally quadrilateral cross-sectional profile isevident, in that four sides can be discerned, although the small rightside is not completely straight. The quadrilateral shape could moreaccurately be characterized as trapezoidal in nature, because the longerpair of sides (which define the width) are generally parallel to eachother, while the two smaller sides are somewhat angled with respect tothe longer sides and to each other. The inventors believe that when suchindividual fiber bodies are slit from a larger sheet using cuttingblades, the angle or attitude of the blades can define whether thesmaller sides will have an angle such as in a trapezoid (wherein the twosmaller sides will have different angles), parallelogram (wherein thetwo smaller sides, in addition to the two longer sides, will be parallelto each other), or rectangle (opposing sides are equal, and the anglesare all about 90 degrees).

The term “quadrilateral” or “generally quadrilateral” as used hereinshall mean and refer to a cross-sectional profile that has four sides,at least two of which are generally parallel to each other and definethe width dimension of the fiber. The two shorter sides or faces (whichtherefore define the thickness aspect of the fiber) may or may not beparallel to each other. The two shorter sides or faces may not even bestraight but could assume, for example, a concave or convex shape if thefibers were extruded as separate bodies rather than being cut from asheet.

FIG. 5 is a partial cross-sectional view, originally taken at about 200×magnification, of an exemplary individual fiber body of the presentinvention, having 0.19 mm measured thickness. In this enlargedmicrophotograph, the small side is generally perpendicular to the twolonger sides (which are 0.19 mm apart), but there is a slightimperfection at the corners. While sharper corners are preferred,because they are believed by the present inventors to decreasefiber-to-fiber entanglement, some rounding or imperfections due to themanufacturing process are to be expected.

FIG. 6 is a view, originally at about 25× magnification, of the outersurface of an exemplary individual fiber body of the present invention.Exemplary fibers are substantially non-fibrillatable when mixed andsubstantially uniformly dispersed in concrete. Accordingly, there aresubstantially no stress-fractures or discontinuities to be seen in therelatively smooth polymer surface of the fiber, although some surfacestreaking and imperfections due to the extrusion process and/or slittingprocess will be seen under magnification. The present inventors believethat introducing into concrete individual fiber bodies that are notmechanically flattened (to the point of having micro-stress-fracturesover the entire surface) and that are not fibrillatable (reducible intostill smaller fibrils when subjected to mechanical agitation inconcrete) will lead to more uniform dispersing and reinforcingcharacteristics, due to uniform fiber surface area to fiber volumeratios and structural integrity from fiber to fiber. Moreover, thesurface of the fibers of the invention, upon being subjected tomechanical agitation within the aggregate-containing concrete, willattain a desirable surface roughness that will facilitate bonding offibers within the concrete matrix when the concrete is solidified.

FIG. 7 is a view at 25× magnification of the fiber of FIG. 6 after ithas been mixed in concrete for five minutes at twenty-five rpm in a twinshaft mixer (and removed for purposes of illustration herein). Althoughthe fiber surface remains substantially free of micro-stress fractures(e.g., cracks), it will experience a roughening or increased opacity dueto the effect of the aggregate in the concrete mix. At 200×magnification, as shown in FIG. 8, the surface of the fiber, beforebeing introduced into concrete, is substantially free of deformities,the only features being perceived at this level of magnification areslight streaking and imperfections due to the extrusion method used formaking the sheet from which the individual fibers are cut. After beingsubstantially uniformly dispersed in a concrete mix, the fiber, as shownat the same 200× magnification in FIG. 9, does not demonstratesubstantial stress-fracturing or fibrillation. However, a desirablesurface roughening is discernible when viewed at this magnificationlevel. Also, because the polymeric material of the fibers of the presentinvention will be highly oriented, it is not unusual that at highermagnifications there will be evident some small strands sticking outfrom the fiber body, but this can be attributed to having molecularpieces separate from each other, or otherwise to imperfections orscraping and does not constitute substantial fibrillation wherein thefiber body splits into smaller fibril units.

The polymer fiber surfaces of FIGS. 10-12 were all photographed at about900× magnification and evidence major differences between exemplaryfibers of the present invention (FIGS. 10, 11) and amechanically-flattened PRIOR ART fiber (as shown in FIG. 12). FIGS. 10and 11 show the fiber surface, respectively, before and after beingmixed in wet concrete using a twin shaft mixer (having counter-rotatingblades) to attain substantially uniform dispersion of fibers in theconcrete. The extrusion streaking, which is seen in FIG. 10, isdesirably roughened as shown in FIG. 11, but without substantialstress-fracturing or subsurface discontinuities. Even after being mixedin the concrete (which contains sand and coarse aggregate such ascrushed stone or gravel), the surface of the fiber of the presentinvention (FIG. 11) does not develop a micro-stress fractured morphology(e.g., sinewed discontinuities) as seen in the mechanically-flattenedPRIOR ART fiber (FIG. 12), but nevertheless is able to provide adesirably roughened surface and overall integrity as well as to providedesirable bendability characteristics for achieving dispersion of aplurality of individual fiber bodies within the concrete matrix.

As used herein and above, the terms “plurality” of “individual fiberbodies” refer to situations wherein a number of fibers that areidentical in terms of material content, physical dimensions, andphysical properties are introduced into the matrix material. Exemplaryfiber bodies of the invention are substantially free of surface stressfractures and substantially non-fibrillatable when mechanically agitatedwithin the matrix material to be reinforced, and they have a generallyquadrilateral cross-sectional profile along said elongated length,wherein average width is 1.0-5.0 mm. and more preferably 1.3-2.5 mm,average thickness is 0.1-0.3 mm and more preferably 0.05-0.25 mm., andaverage length is 20-100 mm. In preferred embodiments, average fiberwidth should exceed average fiber thickness by at least 5:1 but by nomore than 50:1, and more preferably the width to thickness ratio (forfibers having average length of 20-100 mm) is 5-20 (5:1 to 20:1).

In further exemplary embodiments of the invention, a first plurality ofindividual fibers can be mixed with a second plurality of individualfiber bodies (i.e. comprising different materials, different physicaldimensions, and/or different physical properties in comparison with thefirst plurality of fibers) to modify the matrix composition. The use ofadditional pluralities of fibers, having different properties, is knownin the art. Hybrid blends of fibers is disclosed, for example, in U.S.Pat. No. 6,071,613 of Rieder and Berke, and this use of hybrid blendingmay be used in association with the fibers of the present invention aswell. For example, a first plurality of fibers may comprise polymericmaterial having geometry, dimensions, minimum load carrying capacity,and bendability as taught by the present invention, whereas a secondplurality of fibers may comprise another material such as steel, glass,carbon, or composite material. As another example, a first plurality offibers may have a particular bendability characteristic and/or physicaldimension (in terms of average width, thickness, or length), while asecond plurality of fibers may comprise identical or similar polymermaterials and employ a different bendability characteristic and/orphysical dimension(s).

Exemplary pluralities of fibers as contemplated by the present inventionmay be provided in a form whereby they are packaged or connectedtogether (such as by using a bag, peripheral wrap, a coating, adhesive,or such as by partial cutting or scoring of a polymer precursor sheet,etc.). However, as previously discussed above, “individual fiber bodies”of the invention are defined as being themselves separated from otherfiber bodies or as being separable from other fibers when mixed into theconcrete. Thus, exemplary fibers of the invention can be said tocomprise a plurality of individual fiber bodies wherein the individualfiber bodies are separated from each other or wherein individual fiberbodies are connected or partially connected to each other but capable ofbecoming separated after being introduced into and mixed within thematrix composition (to the point of substantially uniform dispersion).

The present inventors believe that the bendability of individual polymerfibers can be controlled more precisely, in part, by using the generallyquadrilateral cross-sectional profile. The present inventors sought toavoid too much flexibility whereby fibers became wrapped around otherfibers (or around themselves) such that fiber balling arises. They alsosought to avoid extreme rigidity, which is often associated withstrength, because this too can lead to undesirable fiber “balling.”Flexibility that is too high (such as in wet human hair) can be just astroublesome as stiffness (such as in the “pick-up-sticks” game played bychildren) because self-entanglement can arise in either case. A highdegree of fiber balling or entanglement means that substantially uniformdispersion has not been attained in the matrix material; and this, inturn, means that the fiber dosage will be inadequate and the materialproperties of the fiber reinforced material will be subject tosignificant variation.

The present inventors believe that for best dispersion properties,bendability needs to be sufficiently high to minimize stress transferamong the other fibers. In order to achieve this, the inventors believedthat alterations in the shape and size of the fiber and elastic modulusof fibers were worth consideration. For example, a lower elastic moduluswill increase the bendability of the fiber, if the shape and size of itscross-section remain constant. On the other hand, inventors also believeit is necessary to consider the elastic modulus of the matrix materialto be reinforced. For polypropylene fibers, the elastic modulus is inthe range of 2-10 Giga Pascals; and for a matrix material such asconcrete (when hardened) the elastic modulus is in the range of 20 to 30Giga Pascals, depending on the mix design used. The present inventorsbelieve that to improve the properties of the matrix material (hardenedconcrete) especially at small crack openings or deflections, the elasticmodulus of the fiber should preferably be at least as high as theelastic modulus of the matrix material (hardened concrete). As mentionedabove, an increase in elastic modulus usually means a decrease inbendability, which has a negative impact on dispersion properties of theplurality of fibers. Thus, in order to keep the bendability high, thepresent inventors have chosen to modify the both the shape andcross-sectional area of the individual fiber bodies. Fracture tests ofconcrete specimens containing the fibers have indicated that a minimumload-carrying capacity under tension (and not minimum tensile stress) offibers is needed for transferring significant stresses across a crackedsection of concrete. This also helps to keep the number of fibers perunit volume of concrete down, and this lowered dosage requirement has apositive effect in terms of improving workability of the fresh fiberreinforced concrete. It is a well-known fact that micro-fibers (havingdiameters of 20-60 micrometers) which are added to concrete for plasticshrinkage cracking control (rather than structural reinforcement, forexample) can not be added in large volumes due to the high number offibers per unit weight (e.g., high surface area). Typical dosage ratesfor these fibers range from 0.3 kg/m3 to 1.8 kg/m3 (0.033 vol. % to 0.2vol. %). Fibers added at these low dosage rates do not have asignificant effect on the hardened properties of concrete. Fibers thatare supposed to have an effect on the hardened properties of concreteneed to be added in larger volumes due to the significant higherstresses needed to be transferred across cracked concrete sections.

Ideally, the present inventors believe that fibers, used in a concretestructure that is cracked, provide a balance between anchoring inconcrete and pull-out from concrete. In other words, about half of thefibers spanning across the crack should operate to pull out of theconcrete while the other half of the fibers spanning the crack shouldbreak entirely, at the point at which the concrete structure becomespulled completely apart at the crack. Thus, exemplary fibers of thepresent invention are designed with particular physical dimensions thatcombine dispersibility with toughness for the purpose at hand.

An exemplary process for manufacturing fibers of the inventioncomprises: melt extruding a synthetic polymeric material (e.g.,polypropylene, polypropylene-polyethylene blend) through a dye to form asheet; cooling the extruded polymer sheet (such as by using a chilltake-up roll, passing the sheet through a cooling bath, and/or using acooling fan); cutting the sheet to provide separate individual fibers(such as by pulling the sheet through metal blades or rotary knives),whereby a generally quadrilateral cross-sectional profile is obtained(preferably having the average width and thickness dimensions asdescribed in greater detail above); stretching the polymer in thelongitudinal direction of the fibers by a factor of at least 10 to 20and more preferably by a factor of 12-16. After the stretching andcutting steps, the individual fibers can be cut to form individualbodies having average 20-100 mm lengths. Thus, exemplary individualfiber bodies of the invention will have elongated bodies, comprising oneor more synthetic polymers, having an orientation (stretch ratio) in thedirection of the length of the fiber bodies (a longitudinal orientation)of at least 10-20 and more preferably 12-16.

A further exemplary method for making the fibers with generallyquadrilateral cross-sections comprises extruding the polymer orpolymeric material through a four-cornered, star-shaped die orifice,stretching the extruded fibers by a factor of 10-20 (and more preferablyby a factor of 12-16), and cutting the stretched fibers to 20-100 mmlengths. In still further exemplary embodiments, fibers having round orelliptical shapes may be extruded, and, while still at a hightemperature, be introduced between rollers (which optionally be heated)to flatten the fibers into a generally quadrilateral shape (although inthis case the smaller faces of the fibers may have a slightly arched orconvex shape).

In addition to the fiber body embodiments mentioned above, still furtherexemplary fiber embodiments are possible. For example, individual fiberbodies may have a variability of thickness and/or width along individualfiber body length of at least 2.5 percent deviation (and more preferablyat least 5.0 percent deviation) and preferably no more than 25 percentdeviation from the average (thickness and/or width). For example, it maybe possible during cutting of the polymer sheet that the blades can bemoved back and forth so that the width of the fibers can be variedwithin the 20-100 mm length of the individual fiber bodies.

In further exemplary embodiments, individual fiber bodies may compriseat least two synthetic polymers, one of said at least two syntheticpolymers comprising an alkaline soluble polymer disposed on the outwardfiber surface thereby being operative to dissolve when said fiber bodiesare mixed into the alkaline environment of a wet concrete mix.Alternatively, individual fiber bodies may be coated with an alkalinesoluble polymer. When dissolved in the alkaline environment of a wetconcrete mix, the outer surface of the fiber could be increased forimproved keying with the concrete when hardened. An alkaline soluble(high pH) polymer material suitable for use in the present inventioncould comprise, for example, polymers of unsaturated carboxylic acids.

Exemplary fibers of the invention may also be packaged with one or moreadmixtures as may be known in the concrete art. Exemplary admixturesinclude superplastizicers, water reducers, air entrainers, airdetrainers, corrosion inhibitors, set accelerators, set retarders,shrinkage reducing admixtures, fly ash, silica fume, pigments, or amixture thereof. The one or more admixtures may be selected, forexample, from U.S. Pat. No. 5,203,692 of Valle et al., incorporated byreference herein. The fibers may also be coated with wetting agents orother coating materials as may be known to those of ordinary skill inthe concrete industry.

Further features and advantages of the exemplary fibers, matrixcompositions, and processes of the invention may be illustrated byreference to the following examples.

EXAMPLE 1 Prior Art

Prior art fibers having an elliptical shaped cross section were testedin terms of bendability and dispersibility in a concrete mix. Theseelliptical fibers were 50 mm long, 1.14 mm wide, 0.44 mm. thick, and hada Young's modulus of elasticity of 4 Giga Pascal. The “bendability”formula discussed above may be employed, wherein bendability “B” wascomputed as B=1/(3·E·I), and the moment of inertia “I” for ellipses iscalculated by the formula, I_(ellipse)=Pi/64·a·b³, where “a” is half thewidth of the elliptical fiber (major axis of the ellipse, i.e., widestdimension through the center) and “b” is half the thickness of theelliptical fiber (minor axis of the ellipse, i.e. thinnest dimensionthrough the center point of the ellipse). The bending deflection “B” wascomputed to be 17.5 mN⁻¹*m⁻². This fiber is considered a “stiff” fiber.30 minutes were required for introducing 100 pounds of these ellipticalfibers into 8 cubic yards of concrete. The concrete resided in the drumof a ready-mix truck and was rotated at 15 revolutions per minute (rpm).Excessive fiber balling was observed. The elliptical fibers did notdisperse in this concrete.

EXAMPLE 2

In contrast to the prior art elliptical fibers of Example 1, fibershaving a generally quadrilateral cross-section were used. Thesequadrilateral fibers had the following average dimensions: 50 mm long,1.35 mm wide, and 0.2 mm thickness, with a Young's modulus of elasticityof 9 Giga Pascal. The bendability “B” of these fibers was computed inaccordance with the formula, B=1/(3·E·I), wherein the moment of inertia“I” for rectangular cross-section was computed in accordance with theformula, I_(rectangle)={fraction (1/12)}·w·t³, wherein “w” is theaverage width and “t” is the average thickness of the rectangle. Usingthe equation, the bendability “B” was computed as 41.2 mN⁻¹*m⁻². Thisfiber is considered flexible. When 100 pounds of these fibers wereintroduced into 8 cubic yards of concrete, located in a ready-mix truckdrum and rotated at the same rate as in Example 1, a homogeneous fiberdistribution was achieved in just 5 minutes. No fiber balling wasobserved.

EXAMPLE 3

The mechanical properties of the fibers themselves have a huge impact onthe behavior of the fibers in concrete, if there is sufficient bondbetween the fiber and the brittle concrete matrix. If the fibers havenot bonded well to the matrix (e.g. fiber pull-out is the major fiberfailure mechanism observed when the fiber reinforced concrete is brokenapart), then the fiber properties will have minimal impact on thebehavior of the composite material. As mentioned earlier, due to thefiber geometry and dimensional ranges inventively selected by thepresent inventors, sufficient bond adhesion between the matrix material(when hardened) and the fibers can be achieved to obtain, ideally, halffiber failure (breakage) and half fiber pull-out. Therefore, fiberproperties such as elastic modulus of elasticity, tensile strength, andminimum load carrying capacity were selected so as to maintain asclosely as possible the ideal 50:50 balance between fiber pull-outfailure and fiber failure. The optimum mechanical properties of thefibers will highly depend on the strength of the matrix: a higherstrength matrix will require a fiber with a higher elastic modulus,higher tensile strength, and higher minimum load carrying capacity.

All the mechanical tests performed on the fiber itself have to be donein direct tension (i.e., longitudinal direction), which is also the modethe fibers fail when embedded in hardened concrete. (Commerciallyavailable machines for such testing are available from known sourcessuch as Instron or Material Testing Systems). For these mechanicaltests, a fiber filament, usually 100 mm long, is fixed on both ends withspecial fiber yarn grips that do not allow the fiber to slip. The fiberis slightly pre-stretched (less than 2 Newton of load is measured). Aload cell measures the tensile load while the fiber is being pulledapart at a constant rate. Typical rates of loading range from 25 mm/min.to 60 mm/min. The strain is measured using an extensometer, which isclamped onto the sample. Strain is defined as the length change dividedby the initial length (also called gauge length) multiplied by 100 andis recorded in terms of percentage. The initial gauge for themeasurements was set to 50 mm.

FIG. 13 shows various load versus strain curves of fibers with differentcross sectional areas. Fibers with number 1 are thinner than fibers withthe number 2. The letters “A”, “B”, “C” are related to the width of thefibers: “A” is the fiber with the smallest width, while “C” is the fiberwith the largest width. Therefore, the fiber with the smallest crosssectional area is fiber “1A”, while the fiber with the largest crosssection is fiber “2C”.

These curves provided in this example show that a fiber with a smallcross sectional area has a much lower minimum load carrying capacitythan a fiber with a larger cross sectional area. Individual fiber bodiesshould have a minimum load carrying capacity such that a plurality ofthe fibers will cumulatively provide a total load-carrying capacityexceeding the tensile stress at which the concrete matrix materialfailed (i.e. the typical stress at failure for the concrete matrix issomewhere in the range of 2 to 5 Mega Pascals). The inventors believethat a minimum load carrying capacity (in tension) of the fiber isnecessary in order to transfer stresses effectively as well as keepingthe number of individual fibers down. By keeping the fiber numbers down,the workability of the fresh concrete can be maintained.

EXAMPLE 4

FIG. 14 shows the tensile stress versus strain curves of the fibersdescribed in the previous example. “Stress” is defined as the loaddivided by the cross sectional area of the fiber. The slope of theinitial part of the ascending curve is directly proportional to themodulus of elasticity of the fiber material. As mentioned earlier, themodulus of elasticity of the fiber should preferably be as close aspossible to the modulus of elasticity of the matrix material, so as totransfer tensile loads across cracks in the matrix immediately afterthey have been initiated. On the other hand, a higher elastic modulusdecreases bendability (i.e. increases stiffness) of the fibers; theinventors discovered that this diminishes the dispersibility of fibersin wet concrete. To minimize the adverse effect of a high elasticmodulus on the bendability of the fiber, the inventors selected agenerally quadrilateral cross-sectional profile and selected a thinnerand wider fiber.

The stress-versus-strain curves shown in FIG. 14 indicate that theelastic moduli and tensile strengths of the different fiber samples areapproximately the same (up to around 7% strain). However, as shown inFIG. 16, the use of different cross-sectional dimensions had a profoundeffect on the performance of the different fiber samples in theconcrete.

EXAMPLE 5

The effect of different geometries of the fibers, as well as differentminimum load carrying capacities on the mechanical properties of fiberreinforced concrete, can be studied using fracture tests. The basicprinciple of a fracture test performed on a given material is to subjecta specimen (in this case the fiber reinforced concrete) to a load thatinitiates cracking in a controlled manner, while measuring the appliedload and the deformation and eventual crack opening of the specimen. Asuitable test for concrete is the Wedge Splitting Test, which is basedon a modified Compact-Tension specimen geometry. The test set-up isdescribed in the Austrian Patent AT 390,328 B (1986) as well as in theAustrian Patent AT 396,997 B (1996).

FIG. 15 depicts a typical uniaxial wedge splitting test device that canbe used for measuring load on concrete materials. A notched cube-shapedconcrete specimen resting on a linear support (which is much like a dullknife blade is split) with load transmission equipment situated in arectangular groove extending vertically down into the top of the sampleconcrete specimen. The load transmission equipment consists of a slimwedge (a) and two load transmission pieces (b) with integrated needlebearings. The crack mouth opening displacement (CMOD) is measured by twoelectronic displacement transducers (Linear Variable DifferentialTransducer or “LVDT” gauges) located on opposing sides of the crack.Both LVDTs (d) are mounted in a relatively simple way on a CMODmeasurement device (c) that is attached to the specimen with screwbolts.

The crack initiates at the bottom of the starter notch and propagates ina stable manner from the starter notch on top of the concrete sample tothe linear support below the sample. To obtain aload-versus-displacement curve, the two crack mouth opening displacementsensors, CMOD1 and CMOD2, and the applied load (downward through thewedge), are recorded simultaneously.

To maintain an approximately constant rate of crack opening, the test isperformed with a rigid testing machine at a constant cross-head speed of0.5 mm/min. to 1.0 mm/min. depending on the wedge angle. The appliedmachine load, F_(M), the vertical displacement, δ_(V), and the crackmouth opening displacement, CMOD, are recorded simultaneously at leastevery second. The fracture energy, G_(F), a measure of the energyrequired to widen a crack, is determined from a load-displacement curveby using the formula$G_{F} = {\frac{1}{B \cdot W} \cdot {\int_{0}^{{CMOD}_{\max}}{{F_{H}({CMOD})} \cdot {({CMOD})}}}}$${\text{with}\quad {CMOD}} = {\frac{1}{2}\left( {{CMOD1} + {CMOD2}} \right)}$

where “B” is the ligament height, “W” is the ligament width (B times Wis the crack surface area), and “F_(H)” is the horizontal splitting loadwhich may be calculated using the following equation,$F_{H} = \frac{F_{M} + {m_{W} \cdot 9.81}}{2 \cdot {\tan \left( {\alpha/2} \right)}}$

wherein “F_(M)” is the applied machine load, “m_(w)” is the mass of thesplitting wedge, and “α” is the wedge angle.

As a measure for the energy for crack initiation, the critical energyrelease rate “G_(Ic)” is calculated (plane stress assumed):$G_{Ic} = {{\frac{K_{Ic}^{2}}{E}\quad \text{with}\quad K_{Ic}} = {k \cdot F_{H,\max}}}$

where “K_(Ic)” is the critical stress intensity factor, which isproportional to the maximum splitting load “F_(H, max)” The constant kdepends on the specimen geometry and can be calculated by a finiteelement program.

The stress factor “K_(I)” is defined as following:

K _(I) =k·F _(H)

where “F_(H)” is the horizontal load measured during the fracture of thespecimen. The stress factor is independent of the specimen size, whichcan be used to compare the behavior of different specimens andmaterials.

The effect of the fiber on the mechanical properties of the compositematerial can be seen after a crack is initiated. FIG. 16 shows thestress-versus-crack opening behavior of different fiber geometries andfiber materials. The larger the area under the curve, the more energythe composite material can absorb while it is being broken apart. Thisphenomenon is also called ‘toughening’ of a material. The higher the‘toughness’ of a material with a certain fiber dosage (volume %), thehigher is the resistance to crack propagation of the material. If acertain fiber achieves similar toughness at a lower dosage, as comparedto other fibers, then such a fiber will be considered to be a moreeffective reinforcing fiber.

FIG. 16 shows that flat, substantially non-fibrillatable fibers of thepresent invention are much more effective when compared to theperformance of fibrillatable fibers of similar dimensions (wheninitially introduced into the concrete) and similar dosage. FIG. 16 alsodemonstrates that the performance of a flat PVA fiber (used at 25%higher dosage rate) with respect to resisting propagation at small crackopenings is slightly better than that of other fibers. However, atlarger crack openings, the exemplary flat fibers of the presentinvention clearly outperformed the flat PVA fiber in resisting higherdeformations.

Further exemplary embodiments of the invention provide synthetic fibers,and matrix materials comprising such fibers, that are particularlysuited for retaining a smooth finish when embedded in matrix materialssuch as concrete. In this respect, the inventors believe that thebendability of the fibers is an important key for obtainingfinishability. Fibers that are not flexible enough tend to pop up againafter the concrete finisher has attempted to smooth out (finish) theconcrete surface.

The inventors believe that finishability is a function of thebendability and the length of the fiber. To achieve the same kind offinishability (wherein fibers do not pop out of a smoothed concretesurface), longer fibers need to be more flexible (i.e., they must have ahigher bendability) than shorter fibers.

For example, a fiber that is 40 mm long, 0.105 mm thick, and 1.4 mm wide(with a Young's modulus of 9.5 GPa) can be observed to have gooddispersion properties and excellent finishibality characteristics.Fibers having a length of 40 mm, a thickness of 0.14 mm, and a width of1.4 mm (with a Young's modulus of 9.5 GPa) showed acceptable dispersionproperties (e.g., a few fiber balls per truck when added in the same wayas the previous more bendable fiber), but it did not finish as well asthe above mentioned fiber. When similar fibers, having length of 50 mm,are added to a concrete ready-mix truck, more fibers sticking out of thesurfacecan be seen despite having the same bendability.

It was thus discovered by the present inventors that exemplary fibers asjust described can have excellent toughness properties at differentcompressive strength levels. For example, with 0.5% or 4.6 kg/m3 offibers, a R_(e,3) value of more than 50% can be achieved with concretecompressive strength range between 10 and 35 MPa which is suitable forflooring (measured according to ASTM C 1018 (1997) or JCI-SF 4 on a 150by 150 by 500 mm³ beam). Incidently, the R_(e,3) value represents theductility factor of a fiber-reinforced concrete sample (e.g., beam), andthis may be calculated as a ratio of the equivalent flexural strength(measured after first cracking and at a deflection of 3 mm, wherein thefibers are bridging the crack) divided by the original flexural strengthof the beam (first cracking strength). See ASTM C 1018 (1997)). TheR_(c,3) value was found, for a given dosage of fibers, depended on thestrength of the concrete matrix, particularly higher strength concrete(e.g., in the range above 35 MPa). The fiber cross-section was thenincreased such that the tensile resistance was increased whileappropriate bendability for good finishability and dispersion wasmaintained. The inventors achieved this by increasing width and reducingthickness. The length of the fiber was also adjusted to maximize theR_(c,3) value.

The inventors also discovered that in dry-mix shotcrete a more bendablefiber had a lower rebound value compared to a less bendable fiber. Inother words, the impact of the sprayed material did not bounce off(i.e., rebound from) the surface being sprayed.

Moreover, the inventors believe that the length and bendability offibers greatly affect the finishability of concrete flooring. Longerfibers with the same bendabilty index did not have a finishability equalto the shorter fibers (i.e., they tended to pop up more from theconcrete surface after it was smoothed). The amount of fiber “pop up” isbelieved to be directly related to the amount of elastic energy storedin the fiber which is being pushed below or into the surface of theconcrete by the smoothing motion of the person who is doing thefinishing. The higher the energy needed for pushing fibers into theconcrete surface, the more likely the fibers will pop up again. Inconsidering this relationship, the inventors realized that the storedelastic energy depends partly upon the level of bending restraintbestowed upon the fiber by portions embedded in the concrete material,partly upon the exposed length of the fibers sticking out from thesurface of the concrete surface, and partly upon the bendability of thefibers.

Hence, the inventors surmise that longer fibers will, on average, tendto have greater portions of their length embedded in the concrete mass,thereby providing more restraint at the point of bending. This greaterrestraint will tend to increase the elastic energy stored in the fiberduring the finishing process; and this, in turn, will tend to increasethe incidence of fiber pop-up from the finished surface. However, in thecase of shorter fibers having the same bendability, the embedded lengthis likely to be shorter on average, and, hence, less restraint would beimposed at the point of bending. Therefore, the lower amount of elasticenergy stored in shorter fibers make them less likely to cause fiberpop-up at the concrete surface. In another words, shorter fibers tend topull out and move with the trowel or other surface-finishing device moreeasily at a concrete surface that is being subjected to concretefinishing, and this is believed to be due to the lower restraint of theshorter fibers causing the fiber to lay down with less elastic bendingenergy stored in the fiber.

Therefore, to achieve similar finishability, a longer fiber will need tohave greater bendability to minimize the elastic energy stored in thefiber, which otherwise will tend to force fibers to stick out of thefinished concrete surface.

The modulus of elasticity, also called Young's Modulus, is the constantrelating stress and strain for a linearly elastic material. In practicalterms, modulus of elasticity is a measure of a material's stiffness. Thehigher the modulus of elasticity, the stiffer a material is. Modulus ofelasticity is determined by chemical composition. Modulus of elasticitymay be expressed in terms of pounds per square inch (lb/in²) also interms of MegaPascals (MPa). One (1) MPa is equal to one (1) Newton/mm².

As shown in FIG. 17, a typical stress-strain curve can be used toillustrate physical properties of a material. The number (1) shown inFIG. 17 indicates the slope of the stress-strain curve corresponding tothe elastic nature of the material, and this is referred to as themodulus of elasticity. By definition, the proportional limit which isindicated in FIG. 17 by the number (2) represents the first point atwhich the elastic behavior of the stress-strain curve is non-linear.This point can also be thought of as the limit of elasticity, for beyondthis point the specimen will begin to demonstrate permanent deformationafter removal of the load, due to plastic strain.

The moment of inertia describes the property of matter to resist anychange in rotation. The moment of inertia, I, for an area of aparticular shape (e.g., rectangle, ellipse or circle) may be calculatedusing the appropriate formula:$I_{rectangle} = {\frac{1}{12} \cdot w \cdot t^{3}}$$I_{ellipse} = {\frac{\pi}{64} \cdot a \cdot b^{3}}$$I_{circle} = {\frac{\pi}{64} \cdot D^{4}}$

where “w” represents the length, “t” represents the breadth (ofrectangle), “a” represents the major axis and “b” the minor axis of anellipse, and “D” represents the diameter of a circle.

Bendability of a fiber can be defined as the resistance of the fiber tochange its shape when an external load is applied. A fiber will betermed more bendable if it requires less force to bend it to a certaindegree. The bending flexibility of a fiber is a function of shape,cross-sectional size, and modulus of elasticity. The bendability, B, ofa fiber can be calculated using the formula:$B = \frac{1}{3 \cdot E \cdot I}$

Using the above equation, the bendability, B, of a 1.2 mm wide and 0.38mm thick fiber with an elliptical shaped cross section with an elasticmodulus of 4 GPa is 26.2 mN⁻¹*m⁻². This fiber is considered a stifffiber. When these fibers were added to the concrete in a ready-mix truck(100 pounds of fibers were added to a 8 cubic yard concrete load in 30minutes, while the drum was rotating with 15 rpm), excessive“fiber-balling” was observed and very poor finishibality was observed:The fibers did not disperse in the concrete, but bundles of fibersstayed together.

Another example involves 50 mm long flat fibers that were 1.4 mm wideand 0.2 mm thick with an elastic modulus of 9 GPa. The bendability, B,is 39.7 mN⁻¹*m⁻², using the above equation. This fiber is considered amore flexible fiber. When these fibers were added to the concrete in thesame manner as in the above-described example with the stiff fiber butin just 5 minutes, a few fiber balls were observed. A homogeneous fiberdistribution throughout the concrete mix was achieved due to the moreflexible nature of the fiber. The finishibility improved compared to theprevious example, but still not satisfactory for all applications.

Another example involves 40 mm long flat fibers that were 1.4 mm wideand 0.105 mm thick with an elastic modulus of 9.5 GPa. The bendability,B, is 259.8 mN⁻¹*m⁻², using the above equation. This fiber is considereda highly flexible fiber. When these fibers were added to the concrete inthe same manner as in the above-described example also in just 5minutes, no fiber balls were observed. A homogeneous fiber distributionthroughout the concrete mix was achieved due to the highly flexiblenature of the fiber. Excellent finishibility was consistently achievedwith this fiber.

When the finishability of the fiber A with a bendability of 39.7mN⁻¹*m⁻² was compared to the finishability of a fiber B with abendability of 259.8 mN⁻¹*m⁻², the following observations were made.After the concrete was finished, fiber A tended to pop out of theconcrete surface after the power trowel had pushed them down (theconcrete appeared to have “goose bumps”). On the other hand, when thesame finish was applied to a concrete containing fiber B, the fibersstayed within the concrete surface. The elastic energy stored in thefibers was too small to cause them to pop out of the surface. After thepower trowel finish, nearly no fibers were visible at the concretesurface when the concrete slab was inspected a day later.

For a fiber with optimized dispersion properties, the bendability has tobe high enough to minimize stress transfer among fibers. In order toachieve this, the shape and the size or elastic modulus of the fiber canbe changed. A lower elastic modulus increases the bendability of thefiber, if the shape and the size of the cross section remain unchanged.For polypropylene fibers the elastic modulus is in the range of 3 to 20GPa (for comparison concrete has an elastic modulus of 20 to 30 GPadepending on the mix design used). To improve the hardened properties interms of toughness of fiber reinforced concrete, especially at smallcrack openings (up to 1 mm), the elastic modulus of the fiber preferablyshould be at least as high as or higher than the elastic modulus of thematrix (concrete). As discussed above, a higher elastic modulusdecreases bendability, which has a negative impact on the dispersionproperties of the fibers. To maintain high bendability, the fiber shapeand the cross sectional area have to be changed. Fracture tests showedthat a minimum load carrying capacity under tension (NOT minimum tensilestress) of fibers is required in order to be able to transfersignificant stresses across a cracked section of concrete. This alsohelps to keep the number of fibers per volume percent down, which has apositive effect on the workability of fresh fiber reinforced concrete.It is a well-known fact that microfibers (having diameters of 20 to 60micrometer), added primarily to minimize cracking due to plasticshrinkage in concrete, cannot normally be added in large volumes, onaccount of the high numbers per unit weight ratio. Typical dosage ratesrange from 0.3 kg/m³ to 1.8 kg/m³ (0.03 vol. % to 0.2 vol. %), such thatthe fibers do not significantly affect the properties of the hardenedconcrete. Fibers intended to affect (i.e., reinforce) hardened concretenormally require higher addition volumes to transfer significantstresses across cracks in the concrete.

Exemplary fibers of the invention which are believed to provideexcellent finishability to the surface of hydratable cementitiousmaterials comprise: a plurality of individual fiber bodies having anelongated length defined between two opposing ends and comprising atleast one synthetic polymer, the individual fiber bodies beingsubstantially free of stress fractures and substantiallynon-fibrillatable when mechanically agitated within the matrix materialto be reinforced, wherein, in said plurality of individual fiber bodies,the average bendability of said fiber bodies is 100-2,500 mN⁻¹*m⁻².Preferred high-finishability fibers also have the exemplary properties:a Young's modulus of elasticity of 4-20 Giga Pascals, tensile strengthof 400-1,600 Mega Pascals. Preferably, the individual fiber bodies aresubstantially free of stress fractures and substantiallynon-fibrillatable when mechanically agitated within the matrix materialto be reinforced, the fiber bodies having a generally quadrilateralcross-sectional profile along said elongated length, thereby havingwidth, thickness, and length dimensions, wherein average width is1.0-5.0 mm, average thickness is 0.05-0.2 mm, average length is 20-75mm; and wherein average width preferably exceeds average thickness by afactor of 5 to 50.

Exemplary high-finishability fibers of the invention comprise at leastone synthetic polymer selected from the group consisting ofpolyethylene, polypropylene, polyoxymethylene, poly(vinylidinefluoride), poly(methyl pentene), poly(ethylene-chlorotrifluoroethylene),poly(vinyl fluoride), poly(ethylene oxide), poly(ethyleneterephthalate), poly(butylene terephthalate), polyamide, polybutene, andthermotropic liquid crystal polymers.

Exemplary high-finishability fibers have individual fiber bodies whereinthe average bendability is 150-2,000 mN⁻¹*m⁻². Particularly preferredhigh-finishability fibers are substantially free of stress fractures andsubstantially non-fibrillatable when mechanically agitated within thematrix material to be reinforced, and have a generally quadrilateralcross-sectional profile along their elongated length, thereby havingwidth, thickness, and length dimensions, wherein the average width is1.0 to 3.0 mm; average thickness is 0.05 to 0.15 mm, average length is20 to 60 mm, wherein average fiber width exceeds average fiber thicknessby a factor of 7 to 40.

Further preferred high-finishability fibers of the invention comprise aplurality of individual fiber bodies having an elongated length definedbetween two opposing ends and comprising at least one synthetic polymer,said individual fiber bodies being substantially free of stressfractures and substantially non-fibrillatable when mechanically agitatedwithin the matrix material to be reinforced, the fiber bodies having agenerally quadrilateral cross-sectional profile along said elongatedlength, thereby having width, thickness, and length dimensions whereinthe average width is no less than 1.0 to 3.0 mm, average thickness is0.075 to 0.15 mm, average length is 20 to 60 mm, average fiber width tothickness ratio is 7 to 30, a Young's modulus of elasticity of 4 to 20Giga Pascals, a tensile strength of 400 to 1,600 Mega Pascals, a minimumload carrying capacity in tension mode of 20 to 1,000 Newtons per fiberbody, the fiber bodies preferably also having an average square area tovolume ratio of 10.5 to 42 mm⁻¹; and also preferably having an averagebendability of 150 to 2,500 mN⁻¹*m⁻².

The present invention also provides matrix compositions comprising theabove-described fibers. An exemplary matrix composition may be comprisedof an adhesive, asphalt, composite material, plastic, elastomer,hydratable cementitious materials, or mixtures thereof. Preferred matrixcompositions are hydratable cementitious composition (e.g., concrete,wet-mix and dry-mix shotcrete, dry mortar, mortar, cement paste), andpreferred fibers comprise polypropylene, polyethylene, or mixturethereof. Preferably, the fibers are present in hydratable matrixcompositions in amounts of 0.05% to 2.0% by volume.

The invention provides high finishability fibers as well as cementitiousmaterials containing such fibers. When the fibers are embedded inconcrete, the concrete preferably will have a compressive strength inthe range of 30 to 60 MPa wherein the average R_(e,3) value is 20 to60%, and the concrete will have a finishablity wherein embedded fibersdo not substantially stick out of said concrete (as visually confirmedby naked eye inspection of the surface of the concrete. The averagebendability of the fiber bodies is preferably 100 to 2,500 mN⁻¹*m⁻²; theaverage width is preferably 1.0 to 3.0 mm; the average thickness is0.075 to 0.15 mm; the average length is preferably 20 to 60 mm; thefibers having a Young's modulus of elasticity of 4 to 20 Giga Pascals;and the fibers having a tensile strength of 400 to 1,600.

The invention is also directed to concrete flooring, and particularlyfloor slabs, containing embedded fibers as described above. Suchfiber-embedded cementitious or concrete floors preferably compressivestrength of 15 to 40 MPa, an average R_(e,3) value of 20 to 60%, andfinishability (wherein embedded fibers do not substantially stick out ofthe concrete), the fibers also having an average bendability of 100 to2,500 Mn⁻¹*m⁻², an average width of 1.0 to 4.0 mm, an average thicknessof 0.050 to 0.15 mm, an average length of 20 to 60 mm, a Young's modulusof elasticity of 4 to 20 Giga Pascals; and preferably a tensile strengthof 400 to 1,600 Mega Pascals.

Still further exemplary fibers have a twist shape, for example as aresult of being cut into separate pieces from strands twisted in themanner of a rope or cable.

The present invention is not to be limited by the foregoing examples andillustrations which are provided for illustrative purposes only.

It is claimed:
 1. Fibers for reinforcing a concrete composition,comprising: a plurality of individual fiber bodies having an elongatedlength defined between two opposing ends and comprising at least onesynthetic polymer, said individual fiber bodies having surfaces that aresubstantially free of stress fractures induced by mechanical-flatteningof the fiber bodies between opposed rollers, said individual fiberbodies being substantially nonfibrillatable into smaller fiber unitsafter mixing in wet concrete to the extent necessary to achievesubstantially uniform dispersal of the fibers therein, and said fiberbodies having a generally quadrilateral cross-sectional profile alongsaid elongated length, thereby having width, thickness, and lengthdimensions wherein the average width is no less than 1.0 mm; wherein theaverage width is no more than 5.0 mm; wherein the average thickness isno less than 0.05 mm; wherein the average thickness is no more than 0.2mm; wherein the average length is no less than 20 mm; wherein theaverage length is no more than 75 mm; wherein said fiber bodies have aYoung's modulus of elasticity no less than 4 Giga Pascals; wherein saidfiber bodies have a Young's modulus of elasticity no more than 20 GigaPascals; wherein said fiber bodies have a tensile strength no less than400 Mega Pascals; wherein said fiber bodies have a tensile strength ofno more than 1600 Mega Pascals; wherein said fiber bodies have a minimumload carrying capacity in tension mode no less than 20 Newtons per fiberbody; wherein said fiber bodies have a minimum load carrying capacity intension mode no greater than 1000 Newtons per fiber body; wherein saidfiber bodies have an average square area to volume ratio no less than10.5 mm⁻¹; wherein said fiber bodies have an average square area tovolume ratio no more than 42 mm⁻¹; wherein said fiber bodies have anaverage bendability “B” nor less than 100 mN⁻¹*m⁻²; and wherein saidfiber bodies have an average bendability “B” not more than 2500 mN⁻¹*m²;said bendability “B” of said fibers being determined in accordance withthe formula, B=1/(3·E·I), wherein the moment of inertia “I” for agenerally quadrilateral is computed in accordance with the formula,I={fraction (1/12)}·w·t³, wherein “w” is the average width and “t” isthe average thickness of the generally quadrilateral cross-section. 2.The fibers of claim 1 wherein, in said plurality of individual fiberbodies, said individual fiber bodies are separated from each other. 3.The fibers of claim 1 wherein, in said plurality of individual fiberbodies, said individual fiber bodies are partially separated from eachother but are completely separable when mechanically agitated within thematrix material.
 4. The fibers of claim 1 wherein, in said plurality ofindividual fiber bodies, said at least one synthetic polymer is selectedfrom the group consisting of polyethylene, polypropylene,polyoxymethylene, poly(vinylidine fluoride), poly(methyl pentene),poly(ethylene-chlorotrifluoroethylene), poly(vinyl fluoride),poly(ethylene oxide), poly(ethylene terephthalate), poly(butyleneterephthalate), polyarnide, polybutene, and thermotropic liquid crystalpolymers.
 5. The fibers of claim 1 wherein said fiber bodies comprisepolypropylene in an amount no less than 75% by weight and said fiberbodies comprise polypropylene in an amount up to 100%.
 6. The fibers ofclaim 1 wherein said fiber bodies comprise a blend of at least twopolymers or a co-polymer comprising at least two of said polymers. 7.The fibers of claim 1 wherein said fiber bodies comprise polypropyleneand polyethylene.
 8. The fibers of claim 1, wherein said fibers areembedded in concrete, said concrete having compressive strength in therange of 15 to 40 MPa wherein the average R_(c,3) value is between 20and 60%, said concrete having finishability whereby said embedded fibersdo not substantially pop out of said concrete.
 9. The fibers of claim 1having a twist shape.
 10. Fibers for reinforcing a concrete composition,comprising: a plurality of individual fiber bodies having an elongatedlength defined between two opposing ends and comprising at least onesynthetic polymer, said individual fiber bodies having surfaces that aresubstantially free of stress fractures induced by mechanical-flatteningof the fiber bodies between opposed rollers, said individual fiberbodies being substantially nonfibrillatable into smaller fiber unitsafter mixing in wet concrete to the extent necessary to achievesubstantially uniform dispersal of the fibers therein, and said fiberbodies having a generally quadrilateral cross-sectional profile alongsaid elongated length, thereby having width, thickness, and lengthdimensions wherein the average width is no less than 1.0 mm; wherein theaverage width is no more than 5.0 mm; wherein the average thickness isno less than 0.075 mm; wherein the average thickness is no more than0.15 mm; wherein the average length is no less than 20 mm; wherein theaverage length is no more than 75 mm; wherein said fiber bodies have aYoung's modulus of elasticity no less than 4 Giga Pascals; wherein saidfiber bodies have a Young's modulus of elasticity no more than 20 GigaPascals; wherein said fiber bodies have a tensile strength no less than400 Mega Pascals; wherein said fiber bodies have a tensile strength ofno more than 1600 Mega Pascals; wherein said fiber bodies have a minimumload carrying capacity in tension mode no less than 20 Newtons per fiberbody; wherein said fiber bodies have a minimum load carrying capacity intension mode no greater than 1000 Newtons per fiber body; wherein saidfiber bodies have an average square area to volume ratio no less than10.5 mm⁻¹; wherein said fiber bodies have an average square area tovolume ratio no more than 42 mm⁻¹; wherein said fiber bodies have anaverage bendability “B” not less than 100 mN⁻¹*m⁻²; and wherein saidfiber bodies have an average bendability “B” not more than 2500mN⁻¹*m⁻²; said bendability “B” of said fibers being determined inaccordance with the formula, B=1/(3·E·I), wherein the moment of inertia“I” for a generally quadrilateral is computed in accordance with theformula, I={fraction (1/12)}·w·t³, wherein “w” is the average width and“t” is the average thickness of the generally quadrilateralcross-section.